1. Field
The present invention relates generally to communication systems, and more specifically to a method and an apparatus for predictive scheduling in a bi-directional communication system.
2. Background
Communication systems have been developed to allow transmission of information signals from an origination station to a physically distinct destination station. In transmitting information signal from the origination station over a communication channel, the information signal is first converted into a form suitable for efficient transmission over the communication channel. Conversion, or modulation, of the information signal involves varying a parameter of a carrier wave in accordance with the information signal in such a way that the spectrum of the resulting modulated carrier is confined within the communication channel bandwidth. At the destination station the original information signal is replicated from the modulated carrier wave received over the communication channel. Such a replication is generally achieved by using an inverse of the modulation process employed by the origination station.
Modulation also facilitates multiple-access, i.e., simultaneous transmission and/or reception, of several signals over a common communication channel. Multiple-access communication systems often include a plurality of subscriber subscriber units requiring intermittent service of relatively short duration rather than continuous access to the common communication channel. Several multiple-access techniques are known in the art, such as time division multiple-access (TDMA), frequency division multiple-access (FDMA), and amplitude modulation multiple-access (AM). Another type of a multiple-access technique is a code division multiple-access (CDMA) spread spectrum system that conforms to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wide-Band Spread Spectrum Cellular System,” hereinafter referred to as the IS-95 standard. The use of CDMA techniques in a multiple-access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE-ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention.
A multiple-access communication system may be a wireless or wire-line and may carry voice and/or data. An example of a communication system carrying both voice and data is a system in accordance with the IS-95 standard, which specifies transmitting voice and data over the communication channel. A method for transmitting data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION”, assigned to the assignee of the present invention. In accordance with the IS-95 standard, the data or voice is partitioned into code channel frames that are 20 milliseconds wide with data rates as high as 14.4 Kbps. Additional examples of a communication systems carrying both voice and data comprise communication systems conforming to the “3rd Generation Partnership Project” (3GPP), embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), or “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems” (the IS-2000 standard).
An example of a data only communication system is a high data rate (HDR) communication system that conforms to the TIA/EIA/IS-856 industry standard, hereinafter referred to as the IS-856 standard. The HDR system is based on a communication system disclosed in co-pending application Ser. No. 08/963,386, entitled “METHOD AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION,” filed Nov. 3, 1997, assigned to the assignee of the present invention. The HDR communication system defines a set of data rates, ranging from 38.4 kbps to 2.4 Mbps, at which an access point (AP) may send data to a subscriber station (access terminal, AT). Because the AP is analogous to a base station, the terminology with respect to cells and sectors is the same as with respect to voice systems.
In a multiple-access communication system, communications between users are conducted through one or more base stations. A first user on one subscriber station communicates to a second user on a second subscriber station by transmitting data on a reverse link to a base station. The base station receives the data and can route the data to another base station. The data is transmitted on a forward link of the same base station, or the other base station, to the second subscriber station. The forward link refers to transmission from a base station to a subscriber station and the reverse link refers to transmission from a subscriber station to a base station. Likewise, the communication can be conducted between a first user on one mobile subscriber station and a second user on a landline station. A base station receives the data from the user on a reverse link, and routes the data through a public switched telephone network (PSTN) to the second user. In many communication systems, e.g., IS-95, W-CDMA, IS-2000, the forward link and the reverse link are allocated separate frequencies.
The design of a multiple-access communication system causes each transmitting subscriber station to act as an interference to other subscriber stations in the network. Therefore, the reverse link capacity is limited by the total interference which a subscriber station experiences from other subscriber stations. The amount of interference is affected by the mode of operation of the communication system—speech, data, or speech and data.
The amount of speech activity at any given moment is non-deterministic. In addition, there is typically no correlation in the level of speech activities among users. Therefore, the total power received at the base station from all transmitting subscriber stations varies over time and can be approximated as a Gaussian distribution. During periods of active speech, the subscriber station transmits at higher power and causes more interference to other subscriber stations. More interference increases the probability of frame errors in the voice data received by the base station. Therefore, the capacity, i.e., the number of users able to have access to the communication system is limited so that only a small portion of the transmitted frames is lost through excessive interference. In contrast, data communication is typically characterized by long period of inactivity, or low activity, punctuated by high bursts of data traffic that drastically decrease the communication system capacity.
Because of the variations in the level of voice activities, and the transmission of data traffic concurrently with the voice traffic, the demand for the reverse link continuously changes over time. To avoid degradation in the quality of the voice communication, the utilization of the reverse link should be dynamically adjusted to match the available reverse link capacity of the base station.
One method of minimizing the interference and maximizing the reverse link capacity is to control the transmit power of each subscriber station. For example, the communication system in accordance with the IS-95 standard utilizes two control loops. The first power control loop adjusts the transmit power of the subscriber station such that the signal quality, as measured by the energy-per-bit-to-noise-plus-interference ratio, Eb/(No+Io), of the signal received at the cell is maintained at a constant level. This level is referred to as the Eb/(No+Io) set point. The second power control loop adjusts the set point such that the desired level of performance, as measured by the frame-error-rate (FER), is maintained. The power control mechanism for the reverse link in IS-95 is disclosed in detail in U.S. Pat. No. 5,056,109, entitled “METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM”, assigned to the assignee of the present invention.
In accordance with another method, each subscriber station transmits at a different bit rate depending on the level of speech activity in the conversation of a user on the subscriber station. A variable rate speech vocoder provides speech data at full rate when the user is actively speaking and at low rate during periods of silence, e.g., pauses. The variable rate vocoder is described in detail in U.S. Pat. No. 5,414,796, entitled “VARIABLE RATE VOCODER,” assigned to the assignee of the present invention.
In yet another method, differences between voice services and data services are utilized. A significant difference between voice services and data services is the fact that the former imposes stringent and fixed delay requirements. Typically, the overall one-way delay of speech frames must be less than 100 ms. In contrast, the data delay can become a variable parameter used to optimize the efficiency of the data communication system. Consequently, the reverse link can be continuously monitored and the data transmission dynamically scheduled so that the reverse link capacity is not exceeded. Several scheduling methods are known in the art. Examples of scheduling methods are disclosed in detail in U.S. Pat. No. 5,914,950, entitled “METHOD AND APPARATUS FOR REVERSE LINK RATE SCHEDULING”, and in U.S. Pat. No. 5,923,650 entitled “METHOD AND APPARATUS FOR REVERSE LINK RATE SCHEDULING”, both assigned to the assignee of the present invention.
Because scheduling method has a profound effect on the performance of a communication system, there is a need in the art for improving scheduling methods and, consequently, utilization of the reverse link.